Air entrainment is an effective means for producing a freeze-thaw durable cementitious composition, such as concrete. The air voids provide spaces for ice crystals to grow thereby relieving internal tensile stresses that can cause cracking of the cementitious composition. In the plastic state of the cementitious composition, air bubbles are entrained by the action of mixing the ingredients and are stabilized by using a surfactant or an air-entraining agent. Therefore, the amount of stable air entrained in cementitious compositions can only be indirectly controlled through the amount or type of air-entraining agent added to the cementitious mixture. If air bubbles of acceptable spacing are not entrained by the mixing action, then the air-entraining agent will be ineffective in supporting the generation of an acceptable air-void structure in the hardened cementitious composition.
Uncertainties in air-void stability and difficulties in achieving consistent spacing of the voids occur quite often in the production of air-entrained cementitious composition, resulting in high production costs for air-entrained concrete. Hence, it is desirable to have an alternative to air entrainment in which void structures are incorporated into cementitious compositions without requiring air bubbles to be stabilized during mixing. This has led to the development of technologies such as those which comprise hollow-core polymeric microspheres as an alternative to air entrainment.
The hollow-core polymeric microspheres are characterized by inner deformable spaces and outer solid shells. The inner spaces of these microspheres are filled with a liquid or a gas, which enables the microspheres to contract and create voids in the concrete when the ambient temperature drops. The expandable microspheres are commercially available under a variety of trade names, such as Expancel®, and use a variety of polymeric materials to form the wall of the microsphere.
The polymeric microspheres that protect concrete from freeze-thaw damage are produced as gas-filled wet-expanded microspheres in a wet foam or slurry form or as gas-filled dry-expanded microspheres in dry powder form. The expanded microspheres have very low densities and they agglomerate in dry powder form. This particle agglomeration is detrimental to performance because the microspheres must be uniformly dispersed throughout the cementitious composition to be effective in protecting the cementitious composition from freeze-thaw damage at a relatively low quantity. The powder form is also difficult to handle as it causes dusting. Therefore, it would be preferred to use the wet-expanded microspheres in slurry form in cementitious compositions. However, the slurry is best produced at the point of addition into the cementitious composition to avoid separation of the very low-density microspheres from the liquid medium during storage. This production limitation of the expanded microspheres in slurry form leads to high production and logistics costs that have stifled introduction of the microsphere technology into practice. A suitable method of delivery of the expanded microspheres in dry powder form into cementitious compositions that eliminates or minimizes particle agglomeration and eases handling of the powder would facilitate introduction of the technology into practice.
No references are found in the prior art which facilitate the use of the technology of the present invention in dry powder form. For example, U.S. Pat. No. 7,435,766B2, by Ong, published on Oct. 14, 2008 is for a Method of delivery of agents providing freezing and thawing resistance to cementitious compositions. Ong teaches the use of microspheres, but the amounts employed vary from that taught by the present invention. Additionally, and most importantly, liquid delivery is used. In contrast, the present invention teaches the use of powder (dry) delivery with a precise mixture/proportion of mineral powders to ensure that the microspheres do not adhere to one another and help to establish a more even dispersion of the microspheres.
Methods of delivery of microspheres developed to date to achieve uniform dispersion in concrete typically involve dispensing the microspheres in some type of liquid. But these have proven to be very costly to implement in practice. To date, the microsphere technology has not been introduced into practice because of the lack of the means by which it can be cost-effectively dispensed into concrete. In contrast, the present invention teaches a means by which microspheres may be cost-effectively dispensed into concrete in a dry powder form such that they are evenly dispersed and effective, unlike the methods found in the prior art.
The invention described herein provides a method to facilitate delivery of dry-expanded polymeric microspheres into a cementitious composition and promote uniform dispersion of the microspheres throughout the cementitious composition.